1. Field of the Invention
The present invention relates to a method for preparing chiral succinates. More particularly, the present invention relates to asymmetric reduction of a 2(E)-alkylidene mono-substituted succinate to produce the corresponding (R)-succinate derivative in good yield and with excellent optical purity. The subject method involves hydrogenation of the 2(E)-alkylidene mono-substituted succinate in the presence of a rhodium complexed (R,R)-enantiomer of a bisphosphine compound represented by the formula: ##STR1## wherein A and B each independently represent substituted and unsubstituted alkyl radicals having from 1 to about 12 carbon atoms, substituted and unsubstituted cycloalkyl radicals having from about 4 to about 7 carbon atoms and substituted and unsubstituted aryl radicals; provided that such radicals provide no significant interference with the steric requirements around the phosphorus atom, and A and B are different.
2. Relevant Art
Rhodium complexed optically active bisphosphine compounds of the formula: ##STR2## are disclosed in U.S. Pat. No. 4,142,992 to W. S. Knowles et al. Utilization of a catalyst of this type, as well as other types of catalysts, for asymmetric reduction of unsubstituted itaconic acid (methylene succinic acid) with 77% ee of the R-enantiomer being produced is disclosed in Asymmetric Synthesis, Chapter 5, J. O. Morrison, Ed., Academic Press, Inc. (1985). See also, W. S. Knowles, Asymmetric Hydrogenation, Acc. Chem. Res., 16, pp. 106-112 (1983), Christopfel et al., J. Am. Chem. Soc., 101: 15, pp. 4406-08 (1979) and Knowles et al, J. Am. Chem. Soc., 97: 9, pp. 2567-68 (1975).
It is known that asymmetric reduction of alpha-acetamidoacrylic acids ##STR3## can be achieved with rhodium complexed optically active bisphosphine compounds. It is also known that although reduction of the unsubstituted olefin, i.e., where R=H, proceeds with 95% ee of the S-enantiomer being produced, the optical purity is not significantly effected, and in some cases is reduced, when R is an aliphatic hydrocarbon such as CH.sub.3 OCH.sub.2 --, CH.sub.3 CH.sub.2 CH.sub.2 --, (CH.sub.3).sub.2 CH--, and the like. See, for example, Scott et al., J. Org. Chem., 46, 5089 (1981). Reduced optical purity is also observed for substituted acetamidoacrylates wherein R is aromatic. See, for example, ApSimon et al., Rec. Adv. Asym. Syn. II, Tetrahedron, 43, p. 5181 (1986).
K. Achiwa, Tetrahedron Lett., 1475 (1978), discloses catalytic reduction of itaconic acid at pressures of 750 psig hydrogen with a catalyst generated in situ from N-acyl-3,3'-bis(diphenylphosphino) pyrrolidine and chloro rhodium octadiene dimer to produce the corresponding S-enantiomer succinic acid with optical purities ranging from 30-83%. Reduction of the sodium salt of itaconic acid with the same catalyst produced the S-enantiomer succinic acid with 92% ee. Ojima et al., Chem. Lett., 567 (1978) and Ojima et al., Chem. Lett. 1145 (1978).
Kawano et al., Tetrahedron Lett., 28, 1905 (1987) disclose reduction of itaconic acid with a ruthenium complex of optically active 2,2'-bis(diphenylphosphino)-1,1,'-binaphthyl to produce the S-enantiomer succinic acid derivative with 88% optical purity. Kawano et al., also disclose reduction of the 1-mono ester and diester with 79 and 68% ee of the S-enantiomer respectively; and reduction of 2-phenylitaconic acid and 3-methoxyphenylitaconic acid to the corresponding S-enantiomer succinic acid derivatives with 90 and 84% ee respectively.